Previously known greenhouses are those in which the climate is controlled by means of ventilation doors or blowers. In those, excess sun energy and excess moisture are removed from the greenhouse by means of ventilation. In optimal growing conditions, the temperature is ca 18-25° C., the air humidity ca 70-90% and the carbon dioxide concentration more than 1000 ppm. Optimal growing conditions require a good control of the air temperature, moisture and carbon dioxide concentration. It is clear that this can not be reached in an open greenhouse. As the cooling takes place by means of external air in an open greenhouse, the temperature of the greenhouse will, especially during the best growing period, rise over the goal. In summer the use of excess carbon dioxide (over the outside level of 350 ppm) does not work as the given carbon dioxide can get out from the greenhouse in connection with the ventilation. An open greenhouse is either not desired in view of the energy consumption. When excess sun energy is ventilated out daytime, heating of the greenhouse is needed at night. In addition, ventilation to remove moisture has to be performed in spring and autumn, which requires additional heating.
The inside air is almost isolated from the outside air in a closed greenhouse. Outside air is not let in from ventilation doors and it is not blown to the greenhouse with blowers but instead, extra heat is lead out technically and also the carbon dioxide needed by the plants is produced technically and its concentration is preferably raised to a level of at least 500-1500 ppm. A closed greenhouse is considered to be the ideal solution for plant growing because the climate can be controlled optimally for the growing of the plants. The use of the closed greenhouse is in first hand restricted by the bad functionality or the high costs of the earlier solutions.
Several international patents have been made for a greenhouse system, wherein the climate control is performed by means of a closed system. WO 00/76296 presents a solution that is based on the use of underground water storages. This solution is possible only in restricted situations as there usually are no underground water basins available. Furthermore, in order to decrease the need of cooling water, water heat accumulators are used in such solutions wherein approximately a half of the daily sun energy is accumulated to be used for the heating of the greenhouse at night. The size of these heat accumulators is however big, for example ca 200 cubic meters for a 1000 square meter greenhouse. The costs required by such a system are of the above reasons considerable and it has not become very general in practice.
In EP patent 0 517 432 A 1, such a heat accumulator is presented to which the daily sun energy is collected and from which a part is taken out during nights for the heating of the greenhouse and a part is lead to the cooler air at night. In this case, the size of the heat accumulator has to be ca 400 cubic meters for a 1000 square meter greenhouse. The big size of the required heat accumulator makes the whole system expensive and the system is not in common use.
References is also made to U.S. Pat. No. 4,044,078 as prior art, which presents an apparatus developed for cooling of storages, in which cold water is sprayed from above trough a grid frame against an air flow and the heated water is cooled with an external cooler. The incidence speed of the air and the water is because of the structure very small, wherefore the apparatus would be very big if used for cooling of greenhouses. Furthermore, the apparatus is not suitable for condensing moisture in the air, because there is only a water inlet but no outlet. The above mentioned facts make the solution not suitable for greenhouse use.
US 2003/0188477 A1 contains a conventional open cooling system for greenhouses, wherein dry outer air is lead to the system which is cooled down along with the evaporation of water of an ambient temperature sprayed to it. Because of the way how the air and water meet, the speed is low, leading to a weak heat exchange. As outer air is blown in the system in to the greenhouse, it is not suitable for cooling of a closed greenhouse. Neither can excess moisture be removed from the greenhouse by means of this method, but the humidification of the air to be blown inside increases the need of removing moisture from the greenhouse by ventilation.
U.S. Pat. No. 4,707,995 comprises a system for the control of the air humidity and temperature of the greenhouse, the function of which is based on the use of salt water for removing moisture. As in the foregoing solution, air is transported through of water spray and the treated water is collected and recovered outside the apparatus. The apparatus is not generally suitable for cooling of greenhouses or removal of moisture.
A similar solution is also presented in JP-publication 4148123 A 19920521. Water is sprayed from above and there are also ventilation devices in the apparatus and the air blown by them is intended to come into heat exchange contact with the sprayed water.
Also in JP-publication 2104222 A 19900417, heat exchange between water and air is used for cooling air in greenhouses. The apparatus comprises a heat exchanger working with cold groundwater with which the greenhouse is cooled from above during nights by means of inlet air and moisture is removed from the lower end of the device. The efficiency of the system is not sufficient to remove daily heat from a closed greenhouse.